1. Field of the Invention
The present invention relates to a power supply apparatus which can supply a regulated (stabilized) DC voltage to electronic apparatuses for industry and livelihood.
2. Description of the Related Art
In recent years, electric and industrial apparatuses have come into wide use, and with this wise use, a high harmonic current is introduced into a power apparatus. This has caused problems such as heating of various apparatuses, malfunction and the like. In order to solve these problems, a suitable measure to the high harmonic is required in various apparatuses. Thus, an AC-to-DC converter has been used as a power supply apparatus built in these apparatuses. The AC-to-DC converter makes use of an energy storage effect by an operation of a reactor and a switching element, and corrects a power factor so as to raise an output voltage.
Japanese Patent Laid-open Publication No. 59-198873 has disclosed a control of the AC-to-DC converter.
As described in the Publication, the control is carried out in the following manner. AC voltage signal is multiplied by a voltage difference between a DC voltage and a target value thereof. Then, a conductive ratio is determined in accordance with the value obtained by the multiply to correct the power factor. FIG. 10 is a circuit diagram showing a construction of the aforesaid conventional power supply apparatus.
An output of an AC power supply 1 is rectified by a rectifier circuit 2 comprising a diode bridge, and then the rectified voltage is switched by a switching element 4 via a reactor 3. Further, voltage across terminals of the switching element 4 is smoothened by a smoothing capacitor 6 via a diode 5 so that a DC voltage is supplied to a load 7. An operation of the switching element 4 is controlled by a switching operation controller 11. The control flow is as follows.
First, a DC voltage value obtained by a DC voltage detecting section 10 is smoothened by a low-pass filter 110. A voltage comparator 112 calculates a voltage difference between the smoothened value and a target value of DC voltage from a target voltage calculating section 111. On the other hand, a target current calculating section 113 calculates a target current by multiplying the voltage difference by a voltage obtained from an input voltage detecting section 9. Next, a current comparator 114 calculates a difference between the target current value calculated by target current calculating section 113 and the current value obtained by the current detecting section 8. Then, in accordance with the difference thus calculated, a conductive ratio of the switching element 4 is determined by a PWM duty calculating section 115. The switching element 4 is switched according to the conductive ratio.
In that AC-to-DC converter, when a load connected to the output terminal becomes large or heavy, as shown in FIG. 11, a ripple having a frequency of two times as much as a voltage 13 of an AC power supply in a DC voltage 14. Therefore, it is general to use a value obtained by passing through a low-pass filter 110 having a slow response speed (with a fixed cut-off frequency of about 1 Hz as shown in FIG. 12) as a DC voltage value to be used for calculating a voltage difference value.
However, in the conventional AC-to-DC converter having the above construction, when a DC voltage raises in a small or light load state in which a load connected to an output terminal is small or light, since a DC voltage passing through a low-pass filter with a slow response speed is used to calculate a voltage difference between the DC voltage and a target value, that is, to determine a conductive ratio of the switching element, there is a problem that the DC voltage continues to raise.
In order to solve this problem, when the DC voltage value raises more than a threshold value which is pre-determined based on a target value, there is a control such that an operation of the switching element is temporarily stopped until the DC voltage value becomes the target value. However, according to such a control, as shown in FIG. 13, there is a problem that a DC voltage 16 is hunting between a target value 18 and a threshold value 19 (In FIG. 13, a waveform 17 shows a rectified voltage value after passing through the low-pass filter.).